The pattern of switching of Schizosaccharomyces pombe (S. pombe)cells is nonrandom when assayed by single cell pedigrees. After two consecutive asymmetric cell divisions, one in four granddaughter cells undergoes a mating-type switch. Previously, we showed that this pattern is due to mat1 imprinting that marks only one sister chromatid in a strand-specific manner, and is related to a site-specific, double-stranded DNA break at mat1. We now show that this imprint is a strand-specific, alkali-labile DNA modification at mat1. The DNA break is an artifact, created from the imprint during DNA purification. We also proposed and tested the model that mat1 is preferentially replicated by a centromere-distal origin(s), so that the strand-specific imprint occurs only during lagging-strand synthesis. Altering the origin of replication, inverting mat1, or introducing an origin of replication affects the imprinting and switching efficiencies in predicted ways. Two-dimensional gel analysis confirmed that mat1 is preferentially replicated by a centromere-distal origin(s). Thus, the DNA replication machinery may confer different developmental potential to sister cells. Our recent work has discovered the biochemical functions of the swi1 and swi3 genes. We found that swi1p and swi3p perform imprinting by pausing and terminating DNA replication at mat1. Our work shows that: 1) the factors swi1p and swi3p act by pausing the replication fork at the imprinting site, and 2) swi1p and swi3p are involved in termination at the mat1-proximal polar-terminator of replication (RTS1). We performed a genetic screen to identify these termination factors and identified an allele that separated the pausing/imprinting and termination functions of swi1p. Our results suggest that swi1p and swi3p promote imprinting in novel ways, both by pausing replication at mat1 and by terminating replication at RTS1. We also showed that Swi1 and Swi3 proteins form a complex in vivo and that both proteins bind to the RTS1 and the mat1 replication pause sites on the chromosome. Future studies will be designed to define the mechanism of imprinting. We have already identified a large number of mat1 mutations that affect imprinting. Molecular analysis of these mat1 mutations should help us define the mechanism of imprinting. We have been looking for another system where such a mechanism of asymmetric cell division operates. Also we are interested in applying our model to hitherto unexplained phenotypes of development in eukaryotes at large. For technical reasons, no studies have been initiated to determine the existence of such a DNA strand-based mechanism of asymmetric cell division in any multicellular organism. We have been searching for another system where such a mechanism operates elsewhere. The Schizosaccharomyces japonicus fission yeast is highly diverged from the well-studied S. pombe species; their protein orthologs are only 55 percent identical at the amino acid level. Despite evolutionary differences, the DNA strand-specific epigenetic imprint at mat1 initiates the recombination event, which is required for cellular differentiation. Therefore, the S. pombe and S. japonicus mating systems provide the first two examples in which the intrinsic strand asymmetry of the double-helical structure of DNA plus strand-specific imprint installed by the DNA replication process at a single locus constitutes the mechanism of asymmetric cell division. This mechanism is very easy to comprehend because the DNA strands asymmetry provides the physical basis for the sister cells' differentiation in these single-cell, haploid organisms. The fission yeast studies have established the unique mechanism of that strand-specific epigenetic marking can be used to bestow developmental asymmetry upon the two daughter cells that receive the subsequently replicated DNA. We recognized that the mechanism of asymmetric cell division that gives rise to the phenomenon of mat1 switching could also explain the vertebrate developmental differentiation that gives rise to body laterality and asymmetric brain hemispheres development in humans. However, in order for that epigenetic mechanism to work in diploids the marked DNA strand from the two homologous chromosomes will have to be segregated selectively. We proposed the somatic strand-specific epigenetic imprinting and selective sister chromatid segregation (SSIS) mechanism to postulate that certain regions of the genome in higher eukaryotes use the strand marking by epigenetic moiety to be followed by coordinated strand/chromatid segregation as a mechanism to establish developmental symmetry or asymmetry. The SSIS mechanism has been advanced to explain variations of body laterality development due to respective gene mutations and for a case of chromosomal translocations in diverse organisms. The 50 percent penetrance of mouse embryonic lethality due to symmetric visceral organs development in the lrd mouse mutants), the 50 percent penetrance of congenital mirror hand movements disorder due to rad51/RAD51 constitution in humans, and the 50 percent psychoses disorders penetrance in families containing chromosome 11 translocations are other such examples. We propose that he LRD gene in mouse and the rad51/RAD51 constitution in humans function to perform selective chromatid segregation of the relevant chromosome. Although mechanistic details remain unknown for all these systems and require future research, developmental symmetry/asymmetry is proposed in each case to the result of selective segregation of precisely two particulate cellular entities to daughter cells at a critical cell division during embryogenesis. In each case, these entities are probably coincident with the On state of the developmental gene located on non-sister chromatids of a homologous pair of chromosomes. SSIS has likely evolved as one of the mechanisms for accomplishing cellular differentiation and development in diverse organisms.